This invention relates to apparatus for the weighing of objects, and especially such apparatus which is suitable for check-weighing of each of a series or train of objects passed onto and from a weighing platform in rapid sequence.
U.S. Pat. No. 3,800,893 of Joseph D. Ramsay and George R. Weaver, filed Sept. 5, 1972 and issued Apr. 2, 1974 illustrates one form of weighing apparatus especially adapted to the high-speed weighing of a series of objects, such as food packages, as they are moved on a conveyor over a weighing platform. The weighing platform is supported at a distance from the balance point of a counter-balanced beam structure having a parallelogram-like form which permits the weighing platform to move vertically up-and-down as the beam tilts, and which also provides a corresponding up-and-down motion of a pick-off arm, the latter motion being sensed and used as an indication of the mass of an object on the platform. The pivots by which the column supporting the weighing platform is mounted on the beam balance are flexure pivots, which provide torsional restraint about the pivot axes at two different positions along the length of the supporting column, and similar flexure pivots are used for the pivots which join together the other members of the parallelogram structure. These flexure pivots provide a spring-restrained balanced-beam operation, since they produce a restoring torque around their respective axes against which the weight of the weighed object acts. In this specific form of weighing apparatus, electrical signals indicative of the vertical displacement, velocity of motion and acceleration of the platform and column during a portion of a cycle of oscillation of the balance are derived and fed to a computer, which is able to deduce and indicate the weight of each object from this information. Such an arrangement is useful for example, in checking that the package weight is within a predetermined range, and serves in such case as a high-speed check weigher.
While this weighing apparatus has proved very satisfactory for many purposes, in the general type of embodiment shown for example in FIGS. 10 and 11 of the above-identified patent the structure is relatively complex and expensive, and exhibits some temperature sensitivity.
In another form of high-speed check weigher which has been built and tested, there is instead utilized a simpler arrangement of an upper and a lower pivoted linkage, each of which is pivoted at one end about an axis fixed in the frame and at its other end about an axis fixed in the platform-supporting column. Flexure pivots are provided at each frame-fixed axis and at each column-fixed axis to provide restoring torques about these axes, opposing the downward motion of the column. The upper linkage is provided with a counter-balancing weight, on the opposite side of the frame-fixed axis from the column, which again establishes a balanced-beam, spring-restrained construction. The vertical motion of an extension of the upper linkage is sensed by an electrical pick-off, and the weight of an object on the platform is derived from the pick-off signals by means of computer circuitry. For proper operation, the structure is such that the column and weighing platform execute an undamped, or only lightly damped, osciallatory vertical motion of the simple harmonic type in response to changes in weight applied to the platform.
While the latter type of weighing apparatus has been found useful, it is again somewhat expensive and critical to construct and assemble properly, and is somewhat sensitive to changes in ambient temperature. It also is subject to some difficulty in adjusting the structure so that the springs associated with the upper and lower linkage will be at their zero-stress conditions for the same position of the column, as is considered desirable in such apparatus.
It is an object of the present invention to provide a new and useful weighing apparatus which is particularly simple and inexpensive, yet effective, and which enables elimination of some parts and simplification of others.
It is also an object to provide such a system which is easy to assemble and adjust, and which has low sensitivity to ambient temperature changes.